I still doubt it very much. A dealer may have fitted it though, that sort of stuff did regularly happen. I have the GMH Engineering manuals and the actual special order pricing based upon what was actually possible for most of HK-HG. A 10-bolt in a 6cyl appears nowhere. There isn't even a tailshaft listed for it, and there is some obscure stuff in there. Like a V8 3.08 banjo fitted behind a 6cyl with Saginaw 4spd, there are parts and a special tailshaft for it but as far as I know none have ever turned up. GMH would have regarded a V8 banjo as a heavy duty option for a 6cyl - it was fitted behind all 253 and most auto 307 and 308 in HK-HG. They never considered the Salisbury for 6cyl. You can never say never though as some strange one-offs for internal use can turn up, but that could be all it could be, you'd never order one as it wouldn't get past the zone office.
My early July 1968 HK V8 auto GTS had a Salisbury in it since the late 60's and people thought it had always been there too. But it hadn't. The car has a banjo rear floor with banjo brake pipe mount, and the diff had a 1969 cast date on it. Someone must have gotten sick of breaking 2.78 or 3.08 banjos and fitted a 3.08 Salisbury in 1969.
The Saginaw was pretty common across HK-HT though. Every 6cyl HK-HG commercial with 4spd manual ticked got a Saginaw. It wasn't a cheap option though. A HK Belmont ute was $2083.11 in August 1968. It was $145.22 which is a 7% increase in price. A 186S over a 161 was only $124.60 in comparison. Or you could have a 5 litre V8, Saginaw and LSD 10-bolt for $548.05